The present invention relates to a self-recovering current-limiting device with liquid metal and including two electrodes made of solid metal.
Current-limiting devices with liquid metal are used for current limiting in low-voltage networks. Soviet Union Patent Document SU 922 911 A describes a self-recovering current-limiting device containing two electrodes which are made of solid metal and flatly configured toward the interior of the current-limiting device, the electrodes being separated by first insulating bodies which are designed as a cylindrical, pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is abruptly displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short-circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. In German Patent Application DE 40 12 385 A1, vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. According to Soviet Union Patent Document SU 1 076 981 A, the connecting channels of adjacent intermediate walls are staggered relative to each other for improving the limiting characteristics. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular gallium-indium-tin alloys (GaInSn alloys), in contact devices. It is a disadvantage that the known current-limiting devices function only in horizontal positions of use and in those slightly deviating therefrom. A known current-limiting device according to Soviet Union Patent Document SU 1 094 088 A is equipped with intermediate walls in which several connecting channels are formed in a circle around the center axis and with separating walls made of copper which are arranged between the intermediate walls and led outward for cooling the liquid metal. This current-limiting device permits positions of use involving rotations of up to 360xc2x0 about the horizontal center axis and inclinations of up to 50xc2x0 relative to the horizontal, which, however, is rendered possible only in conjunction with the separating walls, which disadvantageously carry a potential, the compression spaces having to be individually filled with liquid metal in a manner requiring too much effort because of these separating walls.
Therefore, an object of the present invention is to provide a current-limiting device with liquid metal, the current-limiting device having an extended range of positions of use and being practical to manufacture and user-friendly.
The present invention provides aA self-recovering current-limiting device including a liquid metal, the device comprising:
a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of solid metal, being rotationally symmetrical with respect to a longitudinal axis, and defining respective hollow spaces therein;
a plurality of pressure-resistant insulating bodies; and
a plurality of insulating intermediate walls interleaved with and supported by the plurality of pressure-resistant insulating bodies and defining a plurality of connecting channels disposed in a circular pattern, the plurality of pressure-resistant insulating bodies and the insulating intermediate walls together defining a plurality of compression spaces disposed between the first and second electrodes, the plurality of compression spaces being interconnected by the plurality of connecting channels and being at least partially filled with the liquid metal;
wherein the respective hollow spaces are each connected to an adjacent respective one of the plurality of compression spaces and wherein a respective volume of each of the hollow spaces and an amount of the liquid metal in the current-limiting device are selected so that an upper one of the first and second electrodes is sufficiently wetted with the liquid metal when the current-limiting device is in a position deviating substantially from a position when the longitudinal axis is horizontal.
The hollow spaces of the electrodes provide additional space serving as a reservoir for liquid metal which, when the position of use changes, is available at a different location, thus serving the sufficient reliability of the current-limiting device. In the horizontal position of use, that is, with the longitudinal axis of the current-limiting device being oriented horizontally, the filling of the hollow spaces and the wetting with liquid metal of the surfaces which are important for the current transfer are equal for both electrodes. When the longitudinal axis is inclined, the hollow space of the electrode which moves upward empties of the liquid metal due to gravity in the measure in which the hollow space of the electrode which moves downward fills with liquid metal until, during further increasing inclination, the hollow space of the lower electrode fills completely with liquid metal and the hollow space of the upper electrode, in the extreme case, completely empties; however, this electrode still being sufficiently wetted with the liquid metal. A corresponding redistribution of the filling quantity takes place in the compression spaces, the plurality of connecting channels arranged per intermediate wall guaranteeing that in all intended positions of use, each intermediate wall is in contact with the liquid metal via at least one of its connecting channels. In each inclined position, in the case of an additional rotation about the longitudinal axis, the rotationally symmetrical design of the current-limiting device does not bring about any effective change in the distribution of the liquid metal in the hollow spaces and in the compression spaces so that during normal operation, a sufficient electrical connection between the electrodes exists in all possible positions of use. The connecting channels which are not in contact with the liquid metal serve, on one hand, for uniformly degassing the compression spaces during the filling of the current-limiting device and, on the other band, for pressure compensation between adjacent compression spaces during and subsequent to a short-circuit event.
The hollow spaces may have, for example, a pot-like, conically tapered design, or a double pot-like cylindrical design. In the latter case, a flat connecting lead can reach through the respective outer hollow space, the connecting lead taking the entire width of the outer hollow space for increasing the dimensional stability, a further opening serving the redistribution of the liquid metal in the outer hollow space during the inclination and rotation of the current-limiting device. A staggered arrangement of the connecting channels of adjacent intermediate walls prevents a long electric arc from burning across all compression spaces and, instead, forces the division into a plurality of effective, limiting partial electric arcs. GaInSn alloys as the liquid metal to be used are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10xc2x0 C. to 2000xc2x0 C. at normal pressure and possesses sufficient electrical conductivity.